A feature of many video display systems is the ability to display a picture-in-picture (PIP), which is a small frame within the main image frame, usually of a scaled down image from a different signal. Other types of secondary images are also possible. For example, some display screens are sized for an image having a 16:9 aspect ratio. However, if such a screen is used to display an image with a 4:3 aspect ratio, the remaining area on the side could be used to display a "stack" of small images. This latter feature is sometimes referred to as a picture-on-picture (POP) display.
For displaying these secondary images, a signal that would otherwise be displayed as the main image is scaled down to a smaller size. Typical ratios of the size of the secondary image to the size of the main image are 1:9 and 1:16. For 1:9 scaling, the vertical size and horizontal size are each reduced to 1/3 of the main image size. Likewise, for 1:16 scaling, the vertical and horizontal sizes are each reduced to 1/4 of the main image size. If the same display system is to provide both levels of scaling, conventional systems use expensive processors to implement different digital filtering functions, or they use different hardware for different filters.